Banker Algorithm for Deadlock Avoidance

Deadlock Avoidance

---

This requires that the system has some information available up front. Each process declares the maximum number of resources of each type which it may need. Dynamically examine the resource allocation state to ensure that there can never be a circular-wait condition.

The system's resource-allocation state is defined by the number of available and allocated resources, and the maximum possible demands of the processes. When a process requests an available resource, the system must decide if immediate allocation leaves the system in a safe state.

The system is in a safe state if there exists a safe sequence of all processes:

Sequence < P₁, P₂, ... P_n > is safe for the current allocation state if, for each P_i, the resources which P_i can still request can be satisfied by

• the currently available resources plus
• the resources held by all of the P_j's, where j < i.

If the system is in a safe state, there can be no deadlock. If the system is in an unsafe state, there is the possibility of deadlock.

Example: consider a system with 12 magnetic tapes and 3 processes (P₀, P₁, and P₂):

available = 3
Process	Maximum Needs	Holding	Needs
P0	10	5	5
P1	4	2	2
P2	9	2	7

Is the system in a safe state? If so, which sequence satisfies the safety criteria?

available = 2
Process	Maximum Needs	Holding	Needs
P0	10	5	5
P1	4	2	2
P2	9	3	6

//Bankers algorithm for deadlock avoidance.
#include<stdio.h>
#include<conio.h>
void main()
{
int n,r,i,j,k,p,u=0,s=0,m;
int block[10],run[10],active[10],newreq[10];
int max[10][10],resalloc[10][10],resreq[10][10];
int totalloc[10],totext[10],simalloc[10];
clrscr();
printf("Enter the no of processes:");
scanf("%d",&n);
printf("Enter the no of resource classes:");
scanf("%d",&r);
printf("Enter the total existed resource in each class:");
for(k=1;k<=r;k++)
scanf("%d",&totext[k]);
printf("Enter the allocated resources:");
for(i=1;i<=n;i++)
for(k=1;k<=r;k++)
scanf("%d",&resalloc);
printf("Enter the process making the new request:");
scanf("%d",&p);

printf("Enter the requested resource:");
for(k=1;k<=r;k++)
scanf("%d",&newreq[k]);
printf("Enter the processes which are n blocked or running:");
for(i=1;i<=n;i++)
{
if(i!=p)
{
printf("process %d:\n",i);
scanf("%d%d",&block[i],&run[i]);
}
}
block[p]=0;
run[p]=0;
for(k=1;k<=r;k++)
{
j=0;
for(i=1;i<=n;i++)
{
totalloc[k]=j+resalloc[i][k];
j=totalloc[k];
}
}
for(i=1;i<=n;i++)
{
if(block[i]==1||run[i]==1)
active[i]=1;
else
active[i]=0;
}

for(k=1;k<=r;k++)
{
resalloc[p][k]+=newreq[k];
totalloc[k]+=newreq[k];
}
for(k=1;k<=r;k++)
{
if(totext[k]-totalloc[k]<0)
{
u=1;break;
}
}
if(u==0)
{
for(k=1;k<=r;k++)
simalloc[k]=totalloc[k];
for(s=1;s<=n;s++)
for(i=1;i<=n;i++)
{
if(active[i]==1)
{
j=0;
for(k=1;k<=r;k++)
{
if((totext[k]-simalloc[k])<(max[i][k]-resalloc[i][k]))
{
j=1;break;
}
}
}

if(j==0)
{
active[i]=0;
for(k=1;k<=r;k++)
simalloc[k]=resalloc[i][k];
}
}
m=0;
for(k=1;k<=r;k++)
resreq[p][k]=newreq[k];
printf("Deadlock willn't occur");
}
else
{
for(k=1;k<=r;k++)
{
resalloc[p][k]=newreq[k];
totalloc[k]=newreq[k];
}
printf("Deadlock will occur");
}
getch();
}



OUTPUT